1. Field of the Invention
The invention relates to a method for producing a flat steel product provided with a metal protective layer by way of hot dip coating, in particular a high-strength flat steel product with a tensile strength of at least 500 MPa or a super high-strength flat steel product with a tensile strength of at least 1,000 MPa.
2. Description of Related Art
Where flat steel products are mentioned below, these are intended to mean any cold- or hot-rolled steel strips, steel sheets, steel sheet blanks or the like, wherein the focus here is in particular on the processing of flat steel products in strip form.
There is an increasing demand for high-strength/super high-strength flat steel products owing to their advantageous combination of strength and formability. This applies in particular to sheet applications in automotive car body construction. The outstanding mechanical properties of such flat steel products are based on a multi-phase microstructure of the material, optionally supported by induced plasticity of austenitic phase fractions (TRIP, TWIP or SIP effect). To obtain such a complex microstructure the flat steel products being discussed here conventionally have significant contents of specific alloy elements, which typically include manganese (Mn), aluminium (Al), silicon (Si) or chromium (Cr). A surface refinement in the form of a metal protective layer increases the resistance of the flat steel products to corrosion and therewith the product life thereof, and also increases their visual impression.
Various methods for applying a metal protective layer are known. These include electrolytic deposition and hot dip coating. In addition to electrolytically produced refinement, hot dip refinement has established itself as an economically and ecologically advantageous method. In the case of hot dip coating the flat steel product to be coated is immersed in a metal molten bath.
Hot dip refinement proves to be particularly cost effective if a flat steel product raw material supplied in the full-hard condition is subjected in a continuous pass to the method steps of cleaning, recrystallization annealing, hot dip coating, cooling, optional thermal, mechanical or chemical post-treatment and winding to form a coil.
The annealing treatment carried out in this way can be used to activate the steel surface. For this purpose a N2—H2 annealing atmosphere with typically unavoidable traces of H2O and O2 is conventionally maintained in the annealing furnace passed through in one continuous pass.
The presence of oxygen in the annealing atmosphere has the disadvantage that the alloy elements (Mn, Al, Si, Cr, . . . ) with an affinity to oxygen and contained in the flat steel product which is to be treated in each case form selectively passive, non-wettable oxides on the surface of the steel, whereby the quality or adhesion of the coating on the steel substrate can be lastingly impaired. Various attempts have therefore been made to carry out the annealing treatment of high-strength and super high-strength steels of the type in question here such that the selective oxidation of the surface of the steel is largely suppressed.
A first method of this kind is known from DE 10 2006 039 307 B3. In this method for hot dip refinement of steels with 6-30% by weight Mn, the flat steel product which is to be hot dip galvanised is bright annealed under particularly reductive atmospheric conditions (low H2O/H2 ratio of the annealing atmosphere and high annealing temperature).
EP 1 936 000 A1 and JP 2004 315 960 A each describe method concepts in which the atmospheric conditions in the continuous furnace are set within certain limits and as a function of the temperature of the flat steel product being processed in each case. The internal oxidation respectively of the alloy elements with affinity to oxygen is to be promoted in this way without FeO being formed on the surface of the flat steel product in the process. A precondition of this, however, is exactly matched interaction between the various influencing factors on the annealing gas-metal reaction, such as annealing gas composition and moisture or annealing temperature. For plant-related reasons these are, as a rule, distributed inhomogeneously over the complete furnace chamber. This inhomogeneity makes it difficult to effectively use these processes on a large industrial scale.
Another possibility of preparation of a flat steel product, carried out during the course of an annealing treatment, for hot dip coating consists in that pre-oxidations are carried out in a continuous annealing furnace, used for annealing, within a pre-heating zone and of the DFF type (“DFF”=Direct Fired Furnace). Flames which have been output by gas burners act directly on the flat steel product to be treated in a DF furnace. Since the burners are operated with an excess of O2 (trimming to an air ratio of λ>1, the oxidation potential of the atmosphere surrounding the flat steel product is adjusted such that a covering FeO layer purposefully forms on the surfaces of the flat steel product. This FeO layer prevents the selective oxidation of the alloy elements, with affinity to oxygen, of the flat steel product. In a second annealing step subsequently carried out in a holding zone the FeO layer is reduced completely again to metal iron.
One approach of this type has been known for a long time from DE 25 22 485 A1. Apart from the effects stated above, the advantage of pre-heating the flat steel product in a pre-heating furnace with a DFF-type construction consists in that particularly high heating rates of the steel strip may be attained, and this significantly reduces the duration of the annealing cycle and can therefore increase the output of the hot dip coating plant coupled to a corresponding continuous furnace. The adjustment of an FeO layer thickness of 20-200 nm, regarded as optimal, in an homogeneous, uniform distribution over the strip width can only be controlled with difficulty, however, by way of trimming of the DFF burner flames. An FeO layer which is either too thin or too thick can lead to wetting and adhesion problems.
Very uniform pre-oxidation owing to direct strip contact with an envelope flame allows what is known as a “DFI booster” (“DFI”=Direct Flame Impingement), as is described in DE 10 2006 005 063 A1. However, the use of such a DFI booster is possible only under certain structural conditions, and these do not exist in many current hot dip coating plant.
Methods are also known from EP 2 010 690 B1 and DE 10 2004 059 566 B3 in which an FeO layer is produced on the surface of the respectively processed flat steel product by feeding 0.01-1 vol. % O2 over a period of 1-10 s into a closed reaction chamber. The installation of such a reaction chamber is possible only in an indirectly heated RTF furnace, however, in which the flat steel product is heated by way of heat radiation (“RTF”: Radiant Tube Furnace).
Finally it is known from US 2010/0173072 A1 that the dew point of the oxidation atmosphere can be adjusted in an annealing furnace by targeted humidification in such a way that the desired inner oxidation of the alloy elements of the respectively processed flat steel product is ensured. The pre-oxidation of the flat steel product is carried out in this case in an indirectly heated furnace of the RTF type.
Against the background of the prior art described above, the object of the invention lay in developing a method with which high-strength and super high-strength steels with significant alloy contents of alloy elements with affinity to oxygen (Mn, Al, Si, Cr, . . . ) may be cost- and resource-effectively hot dip galvanised on a continuously operating plant.